As the method of hydrodealkylation of alkylaromatic compounds to produce more useful aromatic compounds, both thermal and catalytic processes have been utilized so far.
In the thermal processes, it is usually required to conduct the reaction under such a severe condition as a temperature higher than 700.degree. C. and a high pressure. This leads to the necessity of using high temperature endurable, expensive materials, and/or of being equipped with sophisticated quench-means to prevent the temperature rise in the reactor due to the exothermic hydrodealkylation reaction, which requires difficult operation for optimum quenching.
In the catalytic processes, the hydrodealkylation reaction is conducted at a temperature lower than 700.degree. C. by the use of catalysts. However, as far as the inventors of this invention know, the catalytic processes which are known heretofore have various problems or difficulties as described below.
Hydrodealkylation with a catalyst such as chromia or molybdenum oxide supported on alumina or others has been utilized as an industrial process of toluene hydrodealkylation. However, the activity of these catalysts is not always high enough, and, if the reaction temperature or hydrogen pressure is increased in order to increase the conversion, side reactions such as hydrocracking of aromatic ring are also promoted.
It is also known to use the catalyst of transitional metal such as Rh, Pt and Ir supported on alumina or others for catalytic hydrodealkylation. However, these metals are not only very expensive and therefore uneconomical, but the activity is often unstable. Furthermore, these metals have the problem of severe deactivation due to poisoning by sulfur contained in feed oil or deposition of coke.
Although solid acid such as silica-alumina and zeolite is also known as a hydrodealkylation catalyst, there are problems that the activity is not high enough and side reactions such as disproportionation of alkylaromatic compounds and coke formation are remarkable.
Alumina can be used as a catalyst for the hydrodealkylation, but the catalytic activity is low though the selectivity of the reaction is high.
Hydrodealkylation process with activated carbon is known by Japanese Patent Application Laid-Open Specification No. 51-23228. However, because activated carbon adsorbs reactant and product compounds too strongly, large amount of coke is formed during the reaction, which results in low selectivity of the hydrodealkylation reaction and lowering of reactivity in a short period of the reaction.